This invention relates to apparatus for conveying molten metal from a tundish to a thin section caster, and particularly to conveying apparatus for supplying molten metal to the caster at low head pressure and uniform flow across the caster width so as to enable effective maintenance of "closed-pool" type caster operation.
Conventional continuous casters for casting relatively large cross sections such as slabs, blooms or billets are in widespread commercial use in the steel industry. These casters have enabled the attainment of significant energy savings through the elimination of slab, billet or bloom rolling which was formerly required in ingot practice. Further savings could be realized through the casting of even thinner slab or strip size cross sections. Although the hot rolling step would not be eliminated, the time required for heating and rolling would be substantially decreased. Commercial apparatus is available for the casting of such thin sections in the non-ferrous metals industry e.g., for the casting of aluminum. Recently, a funnel-type vertical caster mold has been developed for the casting of thin steel slabs of about 2 inch thickness. It is desirable to be able to cast even thinner sections e.g., 1 inch and thinner and to achieve casting speeds in the range of 400 to 600 inches per minute. To accomplish the latter criteria, casters of the horizontal type such as are used in non-ferrous casting would appear to be required. These casters typically have mold surfaces such as blocks, rolls or belts which move exclusively in the direction of casting and do not oscillate like conventional vertical caster molds. In the past, efforts have been made to cast molten steel in a twin belt caster. These efforts were not successful, in part, because of strip surface problems created due to "open-pool" type caster operation. Open-pool refers to the feeding of molten metal from a nozzle to the space between the belts without completely filling the space between the exit of the nozzle and the belt. To obtain a better surface on the casting, closed-pool operation is desirable where only a small gap of about 0.020 inch or less is maintained between the exit of the nozzle and the belts and a meniscus is formed in the metal between the nozzle and the belt completely closing the space therebetween.
To control and maintain the gap between the nozzle and the belts, it is necessary to allow for limited relative motion between the nozzle and the tundish in as many directions as possible. If the nozzle were rigidly attached to the tundish it would be difficult if not impossible to maintain the small gap required. Thermal expansion of the tundish and other feeding system components causes significant movement during preheating and casting. It is also desirable that the gap between the nozzle and the belts be maintained without applying excessive forces on the nozzle. Another factor results from the requirement of forming and maintaining a meniscus between the nozzle exit and the belts. To form a meniscus the pressure of the molten metal at the nozzle exit should be stable and at a positive low pressure level. Finally, another factor is that the flow of molten metal should be uniform in the sense that recirculation eddies and pockets of gas or negative pressure in the feeding system and in the region where the metal enters the caster are prevented.
U.S. Pat. Nos. 4,544,018; 4,576,218 and 4,627,481 show feeding systems for supplying molten steel to a twin belt caster. The '018 patent shows a feed system having a spherical refractory joint for permitting relative movement between the tundish and the caster due to thermal expansion. This reference also shows a flow passage for the steel adjacent to the tundish with a transition from a circular to rectangular cross section adjacent to the nozzle. Also, this system was intended for use in "closed-pool" type operation with a narrow gap between the pouring spout or nozzle and the caster belts (Col. 1 lines 14-16; Col. 3 lines 20-29 and Col. 9 line 61-Col. 10 line 2). However, the system was not designed for low head pressure operation as evidenced by the fact that all of the metal in the feeding system and the tundish is at a level above the level of the exit portion of the nozzle. The '218 patent is related to the '018 patent and discloses additionally a feeding system with a pair of spherical refractory joints so as to provide additional freedom of movement with respect to the tundish. The '481 patent discloses a feeding system similar to that of the two patents just mentioned. It is also worth noting that the flow passage in the nozzle expands in cross section (FIGS. 1, 9 and 12) in the direction of flow so as to be similar to the cross sectional dimension of the belts or mold of the caster (Col. 1 line 68-Col. 2 lines 1-3). While the reference states that the device has a relatively low overall height (Col. 2 lines 54-61) still all of the metal in the tundish and the feed system appears to be at a higher level than the exit level of the nozzle. None of the three aforementioned references disclose a low head feeding system. U.S. Pat. Nos. 3,568,756 and 3,905,418 disclose compression spring assemblies for providing a resilient joint in a caster feed system. The latter patent also discloses a relatively small bore for feeding molten metal to increase the flow rate and decrease the chance of solidification of molten metal in the bore (Col. 3 lines 44-53). Finally U.S. Pat. No. 4,550,767 discloses a nozzle for feeding molten metal to a roll caster. The nozzle has sides converging in the direction of flow with spacers terminating at least one and one-half times the distance of their length from the exit end of the nozzle (Col. 3 line 11-Col. 5 line 51). The reference teaches that the convergent sides of the nozzle prevent flow separation and recirculation eddies in the nozzle. The convergence angle is said to be within the range of 1 to 45 degrees, preferably 1 to 15 degrees and most preferably 2 to 10 degrees. The reference also states that the location of the spacers, i.e., their termination at a spaced distance from the exit of the nozzle, is important for minimizing the effect of the wake downstream of the spacers on the flow profile. A more uniform flow profile is obtained with the spacers terminating the desired distance prior to the exit of the nozzle.